1. Field of the Disclosure
The present disclosure relates to an audio system including a plurality of units connected via a network.
2. Description of the Related Art
Known audio systems (as in Japanese Unexamined Patent Application Publication Nos. 2016-5269 and 2016-19031) may include a plurality of units connected via a network using an ETHERNET (“ETHERNET” is a registered trademark) AVB (Audio Video Bridging) standard (IEEE802.1 Qav, IEEE802.1 Qat, IEEE802.1 AS, IEEE802.1 BA).
As a technology for converting the sample rate of audio data, an ASRC (Asynchronous Sample Rate Converter) is known which generates audio data at a sample rate asynchronous with the sample rate of original audio data (as in Japanese Unexamined Patent Application Publication Open Nos. 2009-504107 and 6-260887).
In an audio system including a plurality of units connected via an ETHERNET AVB based network, different units performing reception and output of digital radio broadcasting may cause problems.
As an example, an audio system will be described which includes a broadcasting receiving unit 100 and an audio processing unit 200 connected via an ETHERNET AVB based network, as illustrated in FIG. 4.
The broadcasting receiving unit 100 includes a digital tuner 101 configured to receive digital radio broadcasting, a transmission buffer 102, and an ETHERNET AVB controller 103. The audio processing unit 200 includes an ETHERNET AVB controller 201, a reception buffer 202, a DAC 203 (digital-analog converter 203), an amplifier 204, and a processing clock generating unit 205. A speaker 300 is connected to the audio processing unit 200.
In this configuration, the digital tuner 101 in the broadcasting receiving unit 100 is configured to perform an operation for receiving digital radio broadcasting synchronized with clocks of the radio broadcasting station 400, which are contained in digital radio broadcasting for broadcasting, generate audio data at a sample rate Fs+dr (Hz), and store them in the transmission buffer 102. Here, Fs is a normal sample rate of audio data, and dr is an error with respect to the normal sample rate of the sample rate of audio data received from the radio broadcasting station 400. After audio data are stored in the transmission buffer 102, the ETHERNET AVB controller 103 in the broadcasting receiving unit 100 transmits the stored audio data in the audio processing unit 200.
The ETHERNET AVB controller 201 in the audio processing unit 200 is configured to receive audio data transmitted from the broadcasting receiving unit 100 and store them in the reception buffer 202. The DAC 203 is configured to read and analog-convert audio data from the reception buffer 202 in synchronization with clocks at a frequency Fs+da (Hz) generated by the processing clock generating unit 205 and output the converted data to the speaker 300 through the amplifier 204. Here, the processing clock generating unit 205 is a clock generator configured to generate clocks at an oscillation frequency Fs, and da is an error with respect to the oscillation frequency Fs of clocks actually generated in the processing clock generating unit 205.
Ideally, the sample rate Fs+dr of audio data output from the digital tuner 101 is matched with the frequency Fs+da of clocks generated by the processing clock generating unit 205. However, an error da−dr may not be eliminated between the sample rate Fs+dr of the audio data and the frequency Fs+da of clocks generated by the processing clock generating unit 205 because each radio broadcasting station 400 has an inherent and unsettled error in clocks in the radio broadcasting station 400 which operates in synchronization with operations of the digital tuner 101, because shifted clocks of the radio broadcasting station 400 if any are detected in the digital tuner 101 under some reception conditions, or because there is a generation error of the frequency of clocks in the processing clock generating unit 205.
On the other hand, ETHERNET AVB standard (IEEE802.1AS) provides a protocol in which one unit being a ground master transmits a SYNC message on a 125 ms cycle to another unit (being a slave) so that clocks in the units can be synchronized. By using this protocol, the processing clock generating unit 205 in the audio processing unit 200 can match the frequency Fs+da of clocks generated thereby with the sample rate Fs+dr of audio data output from the digital tuner 101 in the broadcasting receiving unit 100.
However, also in this case, until completion of the synchronization between the broadcasting receiving unit 100 and the audio processing unit 200 via a SYNC message on a 125 ms cycle, the error da−dr cannot be eliminated between the sample rate Fs+dr of audio data output from the digital tuner 101 and the frequency Fs+da of clocks generated by the processing clock generating unit 205. The completion of such synchronization between the broadcasting receiving unit 100 and the audio processing unit 200 via a SYNC message on a 125 ms cycle requires a period of at least 125 ms.
Therefore, when 0.125×(da−dr) or a difference between the number of clocks generated by the processing clock generating unit 205 (the number of audio data converted by the DAC 203) in 125 ms and the number of audio data output from the digital tuner 101 in 125 ms is equal to or higher than 1, an event that no audio data is available to be converted by the DAC occurs as indicated by “N/A” in FIG. 5, resulting in a drop out in output audio.